Seakeeping criteria revisited

.


Introduction
Marine vessels operate in environments rich in winds and waves.The ability of these vessels to operate in these environments is technically termed seakeeping (Comstock, E.N. and Keane Jr, 1980;Lloyd, 1998).A vessel's seakeeping is determined by its motion in waves, which may degrade or compromise the performance and safety of humans, machinery or other equipment onboard, hull integrity, and consequently, the vessel's operations and interactions with other technical systems and ecosystems (Duncan et al., 2018a(Duncan et al., , 2018b(Duncan et al., , 2020;;Endrina et al., 2019;Ghaemi and Olszewski, 2017;Niklas and Karczewski, 2021).Such undesirable influences make seakeeping an essential design attribute for vessel service life cycle and there are various seakeeping criteria available to guide and evaluate design decisions.The specification of these criteria encompasses, amongst other aspects, vessel type and mission and operational limitations from motion as well as the performance, comfort and safety of prospective vessel occupants (Couser, 2000;Sariöz and Narli, 2005;Comstock, E.N. and Keane Jr, 1980;Crossland and Johnson, 2001;Ghaemi and Olszewski, 2017;Nordforsk, 1987;Pattison and Sheridan, 2004;NATO, 1997).
Despite the availability of seakeeping criteria, these are not standardised and do not cover all vessel types (Niklas and Karczewski, 2021;Sariöz and Narli, 2005;Zu et al., 2022).This poses challenges, not least for unconventional vessels where available criteria may provide misleading bases for design decisions bringing adverse consequences for safety or performance.Notwithstanding, seakeeping has recently gained attention with the shift to more safety-conscious and efficient design perspectives (IMO, 2017;Lin and Lin, 2019;Petacco and Gualeni, 2020).Factors supporting this shift include comfort onboard passenger vessels, public concerns regarding safe and healthy working conditions for crew, better control of shipping transit times and the increasing losses of containers at sea (Kaup et al., 2022;World Shipping Council, 2022).Addressing these emerging concerns necessitates special consideration as regards vessel seakeeping evaluation, for example, detailed analyses of work decks, activities and operations as well as other end-user needs that may be influenced by its seakeeping (Crossland and Johnson, 2001;Gutsch et al., 2020;McCauley and Matsangas, 2005;Tello et al., 2011).
In order to establish relative significance, per praxis, a naval architect receives several specifications or requirements from the client (ship owners) relating to a vessel's seakeeping, including how the vessel should deliver or measure up to older or sister vessels.To ensure that a vessel is delivered meeting the given specifications, naval architects must evaluate its seakeeping to satisfy these specifications.The choice of criteria used for the seakeeping evaluation could include combinations of existing criteria; however, motivating which criteria to apply for evaluation can be challenging and daunting.Either there are no available criteria relating to the type of vessel whose seakeeping is to be evaluated, or naval architects are unsure of whether the criteria chosen can be applied in a given case.Regardless, it is relevant that criteria choices are motivated and that aspects of vessel type, mission, case scenarios or subsystems that the criteria have been specified for are considered to ensure efficient, safe and sustainable designs.
Thus, as its title connotes, this paper revisits seakeeping criteria, conceptually as well as scientifically and practically.More specifically, the paper catalogues prevailing seakeeping criteria and standards, discusses and clarifies the scientific basis upon which these criteria and standards have been developed, the vessel types and vessel features these criteria address.It also identifies the needs and opportunities for improving and developing seakeeping criteria where they are lacking.
The following subsection presents the study design.Section 2 outlines definitions of key terms and concepts used in the paper.Section 3 provides an overview and sources of seakeeping criteria in the literature.Section 4 discusses the background of the criteria overviewed in Section 3, particularly the limiting values -the standards.Section 5 discusses the findings and highlights needs and opportunities for improvement, further development and research.

Study design
Given the growing concerns about seakeeping evaluation, focus group discussions and semi-structured interviews were organised which motivated and shaped the study presented here.The focus group participants were naval architects.The main goal of the focus group discussions was to identify challenges encountered in seakeeping analyses and evaluations.The challenges identified included a lack of criteria for small vessels, including workboats, a lack of standardised criteria for passengers and crew, complexities and inaccuracies of motion prediction methods and the cost of seakeeping analysis, which may primarily cause it to be assigned a lower priority.Other challenges identified in focus group discussions included discrepancies in the definition of seakeeping amongst naval architects and how vessel owners viewed seakeeping.
The participants of the semi-structured interviews consisted of selected crew members from aboard workboats, including pilots, fairway maintenance and rescue vessels.These interviews aimed to ascertain how they, as crew, assessed the working conditions on these workboats from the seakeeping perspective.Findings from interviews supported literature findings that vessel seakeeping limits crew ability to perform effectively and threatens their safety and well-being (Duncan et al., 2018a;Mccauley and Matsangas, 2005;Wertheim, 1998).However, the crews' most prevalent negative experiences on these workboats corroborate the challenges mentioned in the focus group discussions i.e. the lack of seakeeping criteria for workboats and crew.
Based on insights from the naval architects and crews in the focus groups and interviews, a literature study was initiated which constitutes the primary method of the study presented in this paper.The choice of a literature study as scientific method employed in the construct of this paper was to serve a proven scientific basis supporting the challenges and issues raised in the focus group discussions and interviews for further research and development.Hence, the need to catalogue the prevailing seakeeping criteria in order to identify vessel types lacking criteria for evaluation and unearth the scientific basis in the development of criteria to address the prevailing concerns of performance, safety and well-being regarding seakeeping.The literature study was conducted based on existing knowledge of commonly-used seakeeping criteria, keyword searches in databases e.g.Web of Science and Scopus and tracing relevant literature referenced from sources in database searches.The keywords searched in the databases included combinations of "seakeeping", "seakeeping analysis", "seakeeping evaluation", "seakeeping criteria", "motion criteria", "operability criteria", "operability", "safety", "habitability", "survivability", "crew safety", "crew performance", "ship motion", "motion", "marine vehicles", "marine vessels", "ships", "boats" and "workboats".Relevant papers were selected based on the main objective of this study and themes related to the development of seakeeping criteria and the negative effects of seakeeping.

Definitions
As highlighted in the focus groups and interviews, and what also became obvious from the literature review, there are different conceptions and misconceptions of key terms and concepts within the seakeeping knowledge area.For clarity, this section provides the definitions used in this study of seakeeping, seakeeping criteria, standard, operability, habitability, safety, survivability and vessel system.
Seakeeping could be defined as "the motion and operation of ships in waves" (Saunders, 1965, p. 152) or "the ability of our ships to go to sea and successfully and safely execute their missions, despite adverse environmental factors" (Comstock, E.N. and Keane Jr, 1980, p. 24) or "ship behaviour in rough weather" (Lloyd, 1998).The definitions from these sources prove the complexity of the concept of seakeeping.However, as used here, seakeeping is the ability of a vessel to operate in waves.It is characterised by a vessel's motion that directly or indirectly influences operability, habitability, safety and survivability.
According to the IMO Interim Guidelines on the Second Generation Intact Stability Criteria, a criterion is "a procedure, an algorithm or a formula used for the assessment of the likelihood of a stability failure"; whereas a standard is "a boundary separating acceptable and unacceptable likelihood of a stability failure" (Annex IMO, 2017, p. 4).Comparatively, as used here, seakeeping criteria are motion parameters or mathematical models describing events that degrade vessel operability, habitability, safety or survivability.Furthermore, the acceptable thresholds of each parameter or event by which the seakeeping performance of a vessel is evaluated are standards.
Operability refers to crew ability and willingness to operate a vessel and all its installed equipment in a specified seaway (Karppinen and Aitta, 1986).It generally defines the percentage of time a vessel is operable over a given period in a specified sea environment (Nordforsk, 1987).Most seakeeping criteria are concerned with operability.Criteria for evaluating operability concern the aspects of the vessel system that constructively influence total operational performance.Operability criteria are typically expressed in terms of roll and pitch angles, roll motion, vertical and lateral acceleration, deck wetness, propeller and sonar dome emergence and slamming.The primary concern of operability is whether a task or operation is possible to execute.
Habitability relates to the acceptance of the physical conditions on a vessel expressed in terms of lighting, indoor climate, whole-body vibration, noise level and existing characteristics of accommodation and workspaces that influence efficiency and comfort (ABS, 2016;Grech et al., 2019).However, from the seakeeping perspective, habitability concerns the absence of discomfort from seasickness, shock and vibration (Sariöz and Narli, 2005).Criteria evaluating habitability include vertical acceleration and whole-body vibration directly influencing crew comfort, health, perception or performance.These criteria could also be used to evaluate vessel operability.
The definition of safety, however, as concerns vessel seakeeping could be the condition where transit and operations are devoid of accidents, near misses or incidents.A vessel's motion may compromise the safety of crew and cargo onboard and the integrity of its hull.Events resulting from vessel seakeeping that typically threaten safety onboard include excessive acceleration, pure loss of stability, parametric and synchronous rolling, surf-riding or broaching or dead ship conditions (IMO, 2017;Petacco and Gualeni, 2020).Such threats could result in cargo loss, vessel capsizing, and tipping and sliping of humans as well as cargo, causing crew injuries and accidents.
Vessel survivability is the ability of the hull to remain intact and watertight despite exposure to wave loads as well as its ability to remain floating when compromised.Some design requirements specify, for example, in which sea states (considered extreme) a vessel's hull should remain intact or afloat in waves when a substantial part of the hull is damaged.In vessel survivability evaluations, wave loads and resulting motion and acceleration are used to assess the ultimate strength of the hull structure, as applied in Classification Society Rules (e.g.Bureau Veritas, 2022).Survivability evaluations could also be addressed from the perspective of evaluating vessel stability in waves when damaged.
Vessels are manned by crew aided by technical equipment and machinery to keep the vessel operational.Thus, the term vessel system used here refers to the hull, crew and technical equipment installed onboard.
To the best of the knowledge of the authors, the criteria proposed by Cruikshank and Landsberg and Tasaki et al. have no known direct source in the literature, even though mentioned in the publications of Pipchenko (2011) and Ghaemi and Olszewski (2017).Thus, the criteria proposed by Cruikshank and Landsberg and Tasaki et al. are not covered in detail in this study.Table 1 presents examples of different criteria used for seakeeping evaluation.
Criteria are selected depending on what is evaluated.For example, evaluating a vessel system's operability requires criteria that address crew effectiveness, operational requirements of equipment and the ability to maintain the intended service speed in waves.Thus, given this example, the criteria needed may include, but are not limited to, roll angle, vertical acceleration, slamming frequency, propeller and sonar dome emergence.Likewise, evaluating the habitability of a vessel due to motion exposure requires e.g.criteria for whole body vibration and motion sickness incidence.Table 2 provides an overview of seakeeping criteria sources and summarises the vessel types, aspects and elements addressed in each source.
NORDFORSK (1987) criteria were established as a Nordic benchmark to evaluate and compare different designs for improving vessel operability.These criteria were inspired by a review of the state-of-the-art seakeeping criteria by Karppinen and Aitta (1986) and a summary of selected criteria presented by Karppinen (1987).Though these studies focused on merchant vessels, NORDFORSK also includes criteria for naval vessels, fast small crafts, fishing and offshore vessels.
In contrast, NATO criteria (NATO 2018) apply to vessels designed for naval warfare, including combat and auxiliary ships and high-speed craft.They address the operability of naval vessels from a mission-oriented approach and consider aspects of safety.These criteria consider a refined level of detail on the crew's effectiveness, weapons and sensor systems, launch and recovery of deployable vehicles, naval air operations, replenishment at sea, towed systems and damage control.
SGISC (IMO, 2017;Petacco and Gualeni, 2020) are interim guidelines addressing dynamic stability failure modes in waves that compromise vessel system safety.These dynamic stability failure modes include dead ship condition, excessive acceleration, pure loss of stability, parametric rolling and surf-riding/broaching.SGISC introduces different levels of assessment.These include a first and second vulnerability assessment level, a direct stability assessment and operational guidelines.These levels are not necessarily hierarchical but introduce flexibility in assessment approaches.The second vulnerability, direct stability assessment and operational guidance levels incorporate an Addresses biomechanics and keeping balance and could be defined as any interruption to duties or tasks resulting from motion.

Motion-Induced Fatigue
Being physically or mentally fatigued after motion exposure.

Slamming Frequency
Concerns the influences of slamming on the crew, mission performance, specific ship subsystems and the integrity of the ship hull.

Frequency of Sonar Dome Emergence
Concerns the operations of the sonar mounted on certain specialised vessels.

Frequency of Deck Wetness
Addresses green water on deck which poses a risk to the crew working on the deck and equipment mounted on the open deck.

Frequency of Propeller Emergence
For the propulsion system's efficient operation in rough weather.

Whole-body vibration
Addresses comfort, health, perception and motion sickness.

Global bending moment
Concerns structural strength from wave-induced loads and motion.operability analysis by weighting outcomes in different sea states together.Thus, though SGISC primarily concerns safety, it could also be regarded as concerning operability.
The international standard ISO 2631-1 (ISO 1997) addresses relationships between whole-body vibration and human health, comfort, perception and motion sickness and defines methods for quantifying these relationships.It is a generic standard with traces from research applications in different industries.ISO 2631-1 contributes to characterising the habitability of a vessel.
Owing to occupational accidents, despite design rules on fishing vessels less than 24 m in length, Tello et al. (2011) prescribed seakeeping criteria for conditions that limited the safe operation of fishing vessels.These criteria serve as a decision support system for when fishing operations become dangerous.

Background to prevailing seakeeping criteria and standards
Most seakeeping criteria and standards trace their source to the fullscale trials of Aertssen (1968).These full-scale trials spanned ten years and included seven larger commercial vessels (two trawlers, a cross-channel passenger vessel, two cargo liners, a tanker and an ore bulk carrier).The primary aim was to determine when the crew judged voluntary speed reductions to be necessary and how to apply this knowledge in design applications.Aertssen (1968) study resulted in a set of criteria and prescribed standards, as shown in Table 3.
Unintentionally, voluntary speed reduction became the basis from which most operability criteria for merchant vessels were specified and served as an indicator for when one or more threshold limits were exceeded in rough sea conditions.Reviewers raised concerns about instrumentation and measurements, measure definitions and the influence Aertsson's presence as a researcher might have exerted on operator decision-making processes.Nevertheless, these findings contributed to understanding seakeeping and facilitating good seamanship and are the basis from which most criteria have been developed (e.g.Ferdinande and De Lembre, 1970;Karppinen and Aitta, 1986;Lindemann et al., 1977;Nordforsk, 1987).
The following subsections present significant research contributions to seakeeping criteria and standards in NORDFORSK, NATO STANAG 4154, ISO 2631-1, SGISC, USCGC and the criteria for fishing vessels by Tello et al. (2011).The methods, considerations and assumptions these research contributions have used to arrive at the criteria and standards are also highlighted.Karppinen and Aitta (1986) catalogued existing criteria and standards for evaluating seakeeping in an extensive literature review of 38 research contributions, including Aertssen (1968).Their extensive study was motivated by the lack of generally-accepted standards for vessel motion.They deduced that most standards were specified based on operator experiences.These experiences were captured through questionnaires, surveys, interviews and full-scale experiment observations.As such, they concluded that standards are "partly based on personal opinion and partly on data on the behaviour of a few ships and actions taken by their masters in limiting environmental conditions".Karppinen and Aitta's (1986) literature review became the basis upon which Karppinen (1987) presented a set of seakeeping criteria in the Joint Nordic Project.These were then supplemented with complementary full-scale observations and measurements within the Joint Nordic Project and became adopted in the NORDFORSK criteria compilation, which is outlined in Table 4. Karppinen (1987) compared the full-scale trial results from the Joint Nordic project with RMS values of Aertssen's (1968) prescribed standards and those in literature to suggest the standard for vertical acceleration at the forward perpendicular (FP) for merchant ships.The standard 0.275 g for vertical acceleration at the FP of naval vessels was chosen on the basis that it was often used in naval ship operability comparisons e.g. in Bales (1981) and Walden and Kopp (1985); these studies aimed to specify the minimum freeboard requirements for dry fore decks in design.This standard (0.275 g) also agreed with the full-scale trials in the Joint Nordic Project (Karppinen, 1987).The standard 0.65 g for vertical acceleration at the FP of small fast craft was based on consideration of the higher frequency of impact acceleration observed on such vessel types.The motivation for this value (0.65 g) was that humans adapt better to high-frequency than to low-frequency motion.Another conservative consideration for this choice was that such craft are used for shorter periods only.

NORDFORSK
The standard of 0.17 g for vertical acceleration at the FP of RoRo vessels was inspired by the full-scale trial data from Ferdinande and De Lembre (1970) and Lindemann et al. (1977).Ferdinande and De Lembre (1970) performed full-scale trials that studied a car ferry's seakeeping and service performance.Lindemann et al. (1977) performed full-scale hull surveillance on four merchant ship types, including a car-carrier, to improve structural safety in rough weather.From their study, Lindemann et al. (1977) proposed acceptable vertical acceleration and bending from wave loads that would not compromise structural safety.
The lateral acceleration standard of 0.1 g at the bridge for naval vessels and fast small craft was motivated by the US Navy surface ship  criterion for crew performance and safety (Karppinen, 1987).Compared with 0.1 g for naval vessels, a standard of 0.15 g was selected for merchant vessels since masters on such vessels accepted slightly higher lateral acceleration consistent with full-scale observations.Likewise, the roll standards were derived based on full-scale trials supplemented by Cox and Lloyd's (1977) studies on the impairment of personnel behaviour and performance with increasing roll motion.In Karppinen's (1987) study, trade-offs were made in formulating standards for slamming and deck wetness due to limitations in the definitions and availability of full-scale data (from spray to green water).The definition of a slam by Ochi (1964) as dependent on bow emergence and the relative velocity between the bow and waves was employed (Karppinen, 1987).As such, the slamming standards were specified to be consistent with vertical acceleration standards at the FP.Also, deck wetness was defined to occur where the amplitude of relative vertical motion exceeded the vessel's freeboard.Given these limitations, Karppinen (1987) cautioned that careful consideration should be given when applying these standards for slamming and deck wetness for fast small craft.
In addition to the standards presented in Table 4, Karppinen (1987) further prescribed vertical and lateral acceleration, and roll standards addressing crew performance and safety.These standards, shown in Table 5, were specified for different work categories: heavy manual work, light manual work, simple light work, intellectual work and transit of passengers on a ferry or cruise liner.Research studies on the effects of vertical acceleration on human comfort and performance inspired the vertical acceleration standards chosen for these categories.These studies included the contributions of Lawther and Griffin (1986) and Mackay and Schmitke (1978).Mackay and Schmitke (1978) quantified human beings' tolerance to vertical motion using a vibration ride quality index and a tolerance weighting factor; their findings showed that vibration tolerances increased at frequencies below 0.2 Hz.Using surveys and motion measurements, Lawther and Griffin (1986) studied the influence of vertical acceleration on the onset of motion sickness in 4915 passengers on one ferry over 17 crossings on the same route.Each crossing implied up to 6 h of motion exposure.A weighted RMS average of the measured vertical acceleration was calculated to obtain a value that depicted the magnitude of vertical acceleration exposures per voyage.Lawther and Griffin (1986) found that up to an average vertical acceleration RMS of 1 m/s 2 , the acceleration frequencies over a 2-h exposure period ranged between 0.1 and 0.3 Hz with a mean of 0.2 Hz.This range of frequencies resulted in a vomiting incidence of up to 40 %, proving the relationship between vomiting incidence and vertical acceleration RMS.However, Karppinen (1987) cautioned that the standards for roll and lateral acceleration specified in Table 5 are less reliable than those for vertical acceleration since there was less information available on the effects of roll and lateral acceleration on human comfort and performance.NORDFORSK prescribes additional criteria and standards for fishing and offshore vessels, as shown in Table 6.These criteria and standards are based on those for the work categorisations in Table 5.For example, the vertical and lateral acceleration and roll standards at the bridge for offshore and fishing vessels are the same as those for intellectual work (see Table 5), as that type of work is mainly performed on a vessel's bridge.
Smith and Thomas (1989) compiled mission-specific criteria and standards for evaluating naval vessel operability through literature studies.Smith and Thomas' literature studies were motivated by the non-uniformity observed when comparing standards from different sources.They referred to, amongst others, Lain et al. (1979), Walden and Kopp (1985), NORDFORSK (1987) and Karppinen (1987).However, their summary of mission-specific criteria and standards followed the suggestions of Lain et al. (1979) and Walden and Kopp (1985).These criteria and standards, as summarised in Table 7, were dependent on the systematic approach Smith and Thomas (1989) adopted to identify operations or subsystems that limited the fulfilment of missions.
However, Smith and Thomas (1989) made the important observation that, although Lain et al. (1979) discussed and presented criteria and standards for significant subsystems, the standards are "vague as to whether a particular value is really significant or maximum".Nevertheless, Smith and Thomas (1989) suggested mission criteria based on the standards proposed by Lain et al. (1979).Personnel and gun mount standards were specified according to Walden and Kopp (1985), which in turn were specified based on a review of criteria for naval missions specifically applied to DDG51 destroyer variants.
The research performed by Baitis et al. (1984), Colwell (1989), Crossland et al. (1994), Graham (1990) and Graham et al. (1992) further contributed to the standards on motion-induced interruptions using simulators and full-scale experiments.Comstock et al. (1982) proposed criteria and standards for subsystems to compare naval vessels that performed missions requiring air operations; these standards were specified for 100 % effectiveness.Pattison and Bushway (1991), in a literature review of existing criteria, focus group discussions, full-scale

Table 5
Description of different work categories and corresponding standards suggested by Karppinen (1987);adopted in NORDFORSK (1987).measurements and simulation of aircraft operations, proposed criteria and standards for conventional fixed-wing aircraft launch, handling and recovery.Smith (1992) studied the degradation of underway replenishment operations (i.e.connected replenishment, fuelling at sea and vertical replenishment) and proposed standards based on the three primary sources of degradation.These degradation sources are classified as ship-to-ship interactions, equipment limits and human factors.The criteria and standards for ship-to-ship interactions were developed based on the concepts of hydrodynamic linking, relative motion and lateral separation however using several assumptions.However, those for equipment and human factors were inspired by existing standards in literature (Smith, 1992).Sheinberg et al. (2010) suggested improved standards for the US Coast Guard Cutter stern boat ramp deployment criteria.The standards were developed through dedicated sea trials in the Bering Sea by comparing the operational performance of two US Coast Guard Cutters in an earlier study (Sheinberg et al., 2003).In addition, Sheinberg et al. (2010) proposed standards for three safety levels that depend on crew trained experience in launch and recovery and how they occupy a stern boat.Safety level 1 refers to crew standing; level 2, crew sitting and holding; and level 3, crew trained for launch and recovery operations.The standards for these safety levels were derived from public transport study data and adopted into the IMO code of safety for high-speed craft (Sheinberg et al., 2010).An overview of the criteria in the above-referred work is provided in Table 8.

ISO 2631-1
Though referred to as a standard, ISO 2631-1 provides methods for evaluating the effects of human exposure to shock and whole-body vibration.Though commonly referred to as a standard, it is not a standard by the definition in this paper.From a seakeeping evaluation perspective, ISO 2631-1 provides a standardised method for predicting the incidence of motion sickness resulting from exposure to whole-body vibration which is, by definition, a criterion.The development of this predictive model was inspired by, amongst other research, the laboratory studies of Alexander et al. (Alexander et al., 1945a(Alexander et al., , 1945b(Alexander et al., , 1945c)), who examined the influence of different motion parameters on the onset of motion sickness.Additional contributions to its development are the extensive survey and full-scale trials by Lawther and Griffin (1988, 1987, 1986) to determine the onset of motion sickness on ferries.Lawther and Griffin (1987) developed a predictive model for seasickness from their survey findings, which is applied in ISO 2631-1.This model determines the percentage of occupants on a vessel expected to vomit (or reach emesis) within a defined exposure period.Lloyd (1998) andO'Hanlon andMcCauley (1974) developed other motion sickness models also used in seakeeping evaluations.Thus, Lawther and Griffin (1987) compared their model with that of O' Hanlon and McCauley (1974) and reported good agreement.

Tello et al
In order to ensure safe working conditions for crew during fishing operations, Tello et al. (2011) studied the seakeeping performance of eleven fishing vessels with different hull forms in sea states 5 (H s = 3.5 m and T z = 7.5s) and 6 (H s = 5.5 m and T z = 8.5s).These hull forms varied in length from 13.4 m to 45.7 m between perpendiculars.Using the results from their study, Tello et al. (2011) prescribed the standards summarised in Table 9. Parts of these standards are related to absolute motion, and others to motion relative to sea level.However, the standards for acceleration are location-dependent.According to Tello et al. (2011), work deck location considerations are necessary for design and seakeeping evaluations since the effects of derived responses and acceleration depend on the distance between these locations and the vessel's gravitational centre.In suggesting the recommended criteria and standards presented in Table 9, Tello et al. (2011) further referenced Fonseca and Guedes Soares (2002), who also worked towards establishing evaluation criteria for fishing vessels, referring to, amongst others, NORDFORSK (1987) and Bales (1981).

USCGC
USCGC was developed for a range of Coast Guard cutter vessels with the same mission profile and area of operation.These criteria were developed following seakeeping studies using three US Coast Guard vessels in service of different sizes, 47, 82 and 110 feet long (Baitis et al.,   1994).These sizes represented the Coast Guard vessels capable of performing the same missions.Table 10 provides a summary of standards in the USCGC (Baitis et al., 1994).

IMO SGISC
The standards for SGISC were developed based on sample applications of the criteria and knowledge of existing ships.Given the SGISC status as interim guidelines, these standards are tentative, and IMO encourages contributions with sample calculations towards validation (IMO, 2017;Petacco and Gualeni, 2020).Whether it may be advantageous to relate SGISC standards to already-established seakeeping criteria and standards, typically those reviewed in this paper, has been up for discussion in the IMO working and correspondence groups developing these criteria.

Discussion
The review presented in the previous sections is not claimed to be exhaustive.However, it provides a comprehensive overview of seakeeping criteria and standards that may be considered to enjoy widespread recognition and extensive use.Some more novel and emerging criteria such as the IMO SGISC are also described.This section discusses some of the limitations in the field, and the needs and opportunities for further research and development.
As highlighted in Section 4, several researchers attempted to adopt purely scientific methods using observations and measurements in fullscale trials; however, these researchers have been successful with only a few vessels in selected sea environments.The naval architecture community may benefit more if these full-scale trials could cover different sea environments using several of the same type of vessel.Methods for evaluating seakeeping for the crew would also benefit from incorporating Human Factors Engineering e.g.participatory design approaches involving crews (Costa et al., 2015;Mallam et al., 2017).
Applying a critical focus to the standards presented in the tables in Section 5, there are noticeable similarities between the specific standards from different research studies as most research contributions referred to the same sources for standards.Several of the standards specified are unique to specific vessel types, and others are specific to tasks, operations or environmental conditions.As such, before applying these standards in design, it is essential to determine whether they apply to what is under evaluation.Some standards are specified based on educated guesses and assumptions of particular conditions and must be used with caution and consideration.
Furthermore, there are opportunities to improve seakeeping evaluation concerning crew performance, safety and habitability.Even so, opportunities for incorporating health-related evaluations, for example that prolonged exposure to vertical acceleration and whole-body vibration could lead to undesirable health-related issues.Recent studies have aimed to develop and recommend revision of methods to evaluate the musculoskeletal health implications of shock and vibration from exposure to vessel motion (de Alwis et al., 2020; de Alwis and Garme,
In addition, there are new contributions towards a better understanding of motion effects on crew performance using simulator experiments and models of human response to motion (Duncan et al., 2018a(Duncan et al., , 2018b(Duncan et al., , 2020;;Langlois et al., 2009;Matsangas et al., 2014;Schleicher and Blount, 2010).These contributions are motivated because specifying roll, pitch, and vertical and lateral acceleration as criteria relating to the crew or personnel may be misleading (Graham et al., 1992).The effect of motion on human physiology is not limited only by one parameter but a combination of all those listed above (Duncan et al., 2018a;Graham et al., 1992;Langlois et al., 2009;Matsangas et al., 2014).As such, having human response models to motion for seakeeping evaluation would ensure well-tailored criteria for end-users dependent on vessel type and tasks (Zu et al., 2022).Incorporating motion effects on the crew and other vessel occupants in seakeeping evaluations would serve good seakeeping design.
Vessel motion not only degrades health or prevents work onboard but may also compromise crew safety.Crew safety at sea continues to receive increasing attention in order to achieve improvement considering the increasing number of accidents, injuries and loss of life (Bilir et al., 2023;Eliopoulou et al., 2023;Zampeta and Chondrokoukis, 2022).However, safety discussions in the maritime domain are complex due to the varying levels of interest of different stakeholders.Thus, depending on their interest and respective opinions, the concept of safety may extend from preventing loss of life or cargo at sea to being addressed as an economic issue.This complexity has positively and negatively influenced the development of the maritime safety concept (Veiga, 2002).Nonetheless, a common view is required to develop methods of managing the complexities of how safety is defined and approached in the Maritime domain.
The seakeeping criteria reviewed in this paper consider merchant, naval, coast guard, fishing and offshore vessels.However, of particular interest for the project that this study is part of, is the fact that there are no established seakeeping criteria available for the complex category of vessels that may be referred to as workboats i.e. small commercial vessels, such as buoy-tending, fire-fighting, crane, search-and-rescue, hydrographic survey, offshore supply, pipe-laying and pilot vessels IHS Markit, 2018;Maritime & Coastguard Agency, 2018).Workboats are generally fitted with unique technical equipment e.g.crane, pipe-laying or fire-fighting vessels; however, such onboard equipment is rarely taken into account in seakeeping analyses.Recent research contributions have proposed methods for incorporating technical equipment into operability analyses.For example, Ghaemi and Olszewski (2017) propose an operability assessment algorithm that accounts for the vessel system and applies it to a workboat barge with a telescopic boom crane.Also, Gutsch et al. (2020) demonstrated an optimisation of the operability of an offshore construction vessel (OCV) for lifting operations by reducing the crane tip motion.Further, workboats are generally tailored to their mission and the tasks performed by the crew, which involves physical as well as intellectual, and safety-sensitive tasks.Further research on, and development of, seakeeping criteria that address such onboard tasks could obviously enhance the performance and safety of workboats and their crews.
In the field of safety science, there are two schools of thought on safety, as presented by Hollnagel et al. (2015): Safety-I and Safety-II.Safety-I defines safety as "a state where as few things as possible go wrong".Safety-II defines safety as "ensuring that as many things as possible go right".The Safety-I concept is reactive and responds to occurrences; it focuses on human factors as the primary hazard and is fixed on the notion that human failures cause accidents.Safety-II, has a proactive approach, considers systems as a whole, focusing on performance variabilities, and promotes understanding of the successes of events.Safety-I hinders the development of a clear direction towards improving safety in a system since the focus is always on what goes wrong.Conversely, Safety-II promotes an understanding of everyday activities or task performance, focusing on system integration and practice.
In practice, the crew's approach to tasks may deviate from the task descriptions on paper or known procedures due to the influence of vessel motion.Hence, following a Safety-II approach and understanding of how the crew manages their working environment could be essential for enhancing seakeeping evaluations.Hollnagel et al. (2015) propose other considerations towards implementing a Safety-II approach which are worth applying in the development of seakeeping criteria.These include focusing on the frequency of events rather than their severity, accepting the possibility of failure and paying equal attention to efficiency.Thus, identifying the frequency of events onboard e.g.trips, falls, slips, fatigue and allowing a margin to accept these events can guide the specification of criteria and choice of standards without compromising crew performance.Specifying criteria and standards following a Safety-II approach could ensure that transit, operations and onboard tasks are productive and safe despite a vessel's seakeeping characteristics.This approach would form the basis for future research towards improving and developing seakeeping criteria.
A vessel's seakeeping characteristics determine its perceived successes and productivity in its specified operational environment.Certifying that a vessel's seakeeping characteristics will be favourable for maximised productivity without compromising health and safety at sea requires that the seakeeping criteria and standards reflect the vessel type, mission and end-users.

Conclusions
This paper has catalogued the most widespread and extensively-used seakeeping criteria and standards available in the literature, including the US Coast Guard Cutter Criteria, NORDFORSK, NATO STANAG 4154, ISO 2631-1, IMO Second Generation Intact Stability Criteria and criteria by Tello et al. (2011).It further discusses how these criteria and standards have been formulated and what they evaluate.It also identifies opportunities to improve the methods and approaches used in seakeeping evaluation and promotes the active consideration of crew safety Standards for acceleration and motion angles are absolute values.

Table 10
Summary of criteria and standards in the USCGC (Baitis et al., 1994;Stevens and Parsons, 2002).i.The seakeeping criteria reviewed can be used to evaluate operability, habitability, safety and survivability.ii.These criteria are not standardised but vary for different vessel types, depending on the operations and mission of the vessel.These standards are also primarily vessel-type, task or casespecific.iii.Merchant vessels (e.g.container ships, tankers and fishing vessels), naval vessels, and fast small craft, are covered by the criteria reviewed, however there are, for example, no wellestablished seakeeping criteria for workboats.iv.The criteria and standards reviewed have been developed using different approaches including field surveys, focus group discussions, educated guesses, experiments and simulations based on different assumptions.However, a considerable share of the different standards are based on the same limited set of literature and full-scale experiments, which implies a limited knowledge base.v.There are needs and opportunities for improving seakeeping criteria, standards, and evaluation methods, for example concerning crew performance and safety, onboard equipment operations as well as the overall safety philosophy in the maritime domain.
Future studies should aim to delve deeper into devising a method for identifying motion-critical tasks based on vessel type and mission, and based on this method, develop seakeeping criteria that ensure crew performance and safety.
Having appropriate seakeeping criteria and standards for seakeeping evaluations (reflecting vessel type, mission, design and occupants) would ensure safe and efficient vessel design and well-communicated confidence to prospective owners as concerns the abilities of vessels procured.

a
Values interpolated for lengths between perpendiculars in the range 100 m ≤ L ≥ 330 m. b Values interpolated for lengths between perpendiculars in the range 100 m ≤ L ≥ 300 m.

Table 1
Conceptual overview of seakeeping criteria.

Table 2
General overview of seakeeping criteria sources.
Acceleration and deceleration are in double amplitude = 4(RMS).
a Values interpolated for lengths between perpendiculars in the range 100 m ≤ L ≥ 330 m. b Values interpolated for lengths between perpendiculars in the range 100 m ≤ L ≥ 300 m.M.Zu et al.